Supported GPU's not showing
Unfortunately your AMD R9 280 card doesn't fulfill the requirements for GPU rendering in Blender 2.8x. You would either need a


Nvidia graphics card with CUDA compute capability 3.0 or higher
AMD graphics card from GCN second generation or later


The AMD R9 280 has the GCN first generation architecture.


  CUDA
  
  CUDA requires graphics cards with compute capability 3.0 and higher. To make sure your GPU is supported, see the list of Nvidia graphics cards with the compute capabilities and supported graphics cards. CUDA GPU rendering is supported on Windows, macOS, and Linux.





  AMD
  
  OpenCL is supported for GPU rendering with AMD graphics cards. Blender supports graphics cards with GCN generation 2 and above. To make sure your GPU is supported, see the list of GCN generations with the GCN generation and supported graphics cards.
  
  AMD OpenCL GPU rendering is supported on Windows and Linux, but not on macOS.


Please note that the requirements for GPU rendering are higher than the minimum requirements for running Blender.


  These requirements are for basic Blender operation, rendering using the graphics card requires slightly different requirements.